A diagnostic study of interannual variability of Indian summer monsoon using outgoing longwave radiation (OLR) data

ABSTRACT. Using the monthly outgoing longwave radiation (OLR) data obtained from NOAA polar orbiting satellites, during the period 1979-92, composite OLR anomalies in respect of good monsoon years (1983 and 1988), bad monsoon years (1982 and 1987 for the case associated with ENSO and 1979 and 1986 separately for the case without ENSO) and normal monsoon years (1980, 1981, 1984, 1985, 1989, 1990, 1991 & 1992) were examined. The computation has been performed over the global tropics (30°N-30°S) bounded between the longitudes 50°E and 130°W (through date line) on 5° longitude × 5° latitude grid. There are significant differences in the spatial distributions of composite OLR anomalies between these four cases from the month of April to September indicating spatial and temporal changes in the organized convective pattern. For the good monsoon years persistent negative anomalies indicating enhanced convective activity were observed over the Indonesian regions, whereas large positive anomalies indicating depressed convective activity were observed over equatorial Pacific just west of date line. During the bad monsoon years above normal convection was observed over Pacific region (ENSO case) and over equatorial Indian Ocean (Non ENSO case). During normal monsoon years the spatial patterns of OLR anomalies were similar to that of good monsoon years, but with weaker anomalies. These observations can be explained through the relative interaction between tropical convergence zone (TCZ) over the Indian sub-continent and that over the north Indian Ocean and Pacific. The eastward shift of the convective activity during El-Nino years can be attributed to shift/reversal of Walker circulation. There are strong signals of OLR anomalies during pre-monsoon months which may be useful in inferring the nature of the subsequent monsoon activity.

Drift-Alfvén fluctuations and transport in multiple interacting magnetized electron temperature filaments

The results of a basic electron heat transport experiment using multiple localized heat sources in close proximity and embedded in a large magnetized plasma are presented. The set-up consists of three biased probe-mounted crystal cathodes, arranged in a triangular spatial pattern, that inject low energy electrons along a strong magnetic field into a pre-existing, cold afterglow plasma, forming electron temperature filaments. When the three sources are activated and placed within a few collisionless electron skin depths of each other, a non-azimuthally symmetric wave pattern emerges due to interference of the drift-Alfvén modes that form on each filament’s temperature gradient. Enhanced cross-field transport from chaotic ( $\boldsymbol{E}\times \boldsymbol{B}$ , where $\boldsymbol{E}$ is the electric field and $\boldsymbol{B}$ the magnetic field) mixing rapidly relaxes the gradients in the inner triangular region of the filaments and leads to growth of a global nonlinear drift-Alfvén mode that is driven by the thermal gradient in the outer region of the triangle. Azimuthal flow shear arising from the emissive cathode sources modifies the linear eigenmode stability and convective pattern. A steady-current model with emissive sheath boundary predicts the plasma potential and shear flow contribution from the sources.

Constraining convection across the AGB with high-angular-resolution observations

We present very detailed images of the photosphere of an AGB star obtained with the PIONIER instrument, installed at the Very Large Telescope Interferometer (VLTI). The images show a well defined stellar disc populated by a few convective patterns. Thanks to the high precision of the observations we are able to derive the contrast and granulation horizontal scale of the convective pattern for the first time in a direct way. Such quantities are then compared with scaling relations between granule size, effective temperature, and surface gravity that are predicted by simulations of stellar surface convection.

The convective photosphere of the red supergiant CE Tauri

Context. Red supergiant stars are one of the latest stages in the evolution of massive stars. Their photospheric convection may play an important role in the launching mechanism of their mass loss; however, its characteristics and dynamics are still poorly constrained. Aims. By observing red supergiant stars with near infrared interferometry at different epochs, we expect to reveal the evolution of bright convective features on their stellar surface. Methods. We observed the M2Iab-Ib red supergiant star CE Tau with the VLTI/PIONIER instrument in the H band at two different epochs separated by one month. Results. We derive the angular diameter of the star and basic stellar parameters, and reconstruct two reliable images of its H-band photosphere. The contrast of the convective pattern of the reconstructed images is 5 ± 1% and 6 ± 1% for our two epochs of observation. Conclusions. The stellar photosphere shows few changes between the two epochs. The contrast of the convective pattern is below the average contrast variations obtained on 30 randomly chosen snapshots of the best matching 3D radiative hydrodynamics simulation: 23 ± 1% for the original simulation images and 16 ± 1% for the maps degraded to the reconstruction resolution. We offer two hypotheses to explain this observation. CE Tau may be experiencing a quiet convective activity episode or it could be a consequence of its warmer effective temperature (hence its smaller radius) compared to the simulation.

Numerical Simulation of the Convection in a Non-Homogenous Lid-Driven Square Cavity Subjected to a Gravitational Stable Condition

The lid-driven flow inside a porous square cavity is numerically simulated. The porous media is modelled on the microscopic scale (heterogeneous porous medium) with a square heat conductive single block representing the solid constituent. Conversely, the fluid relies between the block and the cavity surfaces. A vertical positive thermal gradient, obtained by keeping the sliding-lid temperature TH higher than the base one TC, aligned with the gravity force enables a gravitational stable condition where the buoyant-induced flow does not occurs spontaneously. Instead, the flow comes about as the cavity top surface slides with constant velocity. Conservation equations are applied separately for each constituent and are coupled by boundary conditions at the fluid to solid interface (block surface). The Boussinesq-Oberbeck approximation accounts for the buoyant effects. The equations are solved via the finite volume method with the use of the SIMPLE algorithm for the pressure-velocity coupling and QUICK interpolation scheme for the treatment of the advection terms. The aim of the present work is to investigate how variations on the flow parameters and the block size affect the thermal process throughout the cavity. A top lid velocity based Reynolds number evaluates the intensity of the forced convection process while the Grashof number is associated with the intensity of buoyancy. The flow parameters cover only the laminar regime, such as 102≤Re≤103 and 103≤Gr≤107. The Re and the Gr numbers are also analyzed by the means of the Richardson number, Ri, which accounts the relative predominance of buoyancy over the inertia effects. Moreover, a clear fluid cavity and enclosure configurations with three different block dimensions, namely B = 0.3, 0.6 and 0.9, are simulated. The heat transfer across the cavity can be characterized as a competitive effect, since the flow is hindered as the buoyancy effect rises. Results show that an increase in Re, or decrease in Gr, enhances the heat transfer, revealing a convection dominant regime. Alternatively, an increase in Gr, or a decrease in Re, leads the fluid to a stagnant-prone condition where a conduction dominant regime is verified. Thus, the surface-average Nusselt number, Nuav, tends to unity as the flow is confined to the adjacency of the sliding-lid. The placement of the single block in the cavity can enhance or hinder the heat transferred, depending on the flow regime. For instance, if a B = 0.6 block is inserted in the presence of a convection dominant regime, the Nuav is increased. Conversely, if the fluid is quiescent, a B = 0.6 block alters the flow path and the Nuav decreases. Intense blockage effects are observed for larger values of B since the block interferes on the flow more significantly. For a convection dominant regime, for instance, a B = 0.9 block causes the Nuav to drop. However, in the presence of stagnant fluid, the same obstacle forces the flow to circumvent it. Thus, the Nuav number increases, indicating that heat transfer mode returns to a convective pattern.